That really depends on your movie structure. The problem with preloaders is that a lot of people are using them as an excuse to over-do their content and not optimize it. Flash is a streaming protocol and the more you can stream without having to preload it, the better. As a rule my flash content is very much Actionscript based so I generally have little to preload, but if I do, I don't allow preloading or any more than about 10 seconds at any one time.
You might consider loading up your larger graphic as an external movie clip. This way you could preload the rest of your clip quickly and then when the user calls that large clip it will preload itself. 2 smaller prealoders are beter than one big one IMHO.
Alternatively, experiment with how much data can be loaded during th duration of your clip. So if you you preload the first 3 of 4 scenes, while those 3 play, does scnee 4 have a chance to load?
If so you don't have to preload them all.
You're right to be streaming at 28.8 though, as many people on modems view more than one page at a time, so you will not have dedicated 56k bandwidth.
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